Am I missing something? Is this being overridden higher in the chain, somehow? I don't really understand how a GUIControl recieves a GUIEvent, or rather.. who sends the GUIEvent in the first place?

I see a comment in _GUIMethodCallbacksClassMethods near onMouseEnter:

////////////////////////////////////////////////////////////////////// THE FOLLOWING SECTION IS FOR METHODS WHICH ARE PASSED DOWN TO// A CONTROL'S ROOT PARENT FOR HANDLING OF EVENTS// args.source will contain the original control which is passing the event down////////////////////////////////////////////////////////////////////

Since E_AbilityBar's root parent is E_AbilityBar->GUIPanel->GUIControl->Scripted, I'd expect to see onMouseEnter handled somewhere, but I don't see it.

HiYes using this function execute whatever code you put in when a mouse is put over the control.

This works fine for me, i put this bit of code into E_AbilityBarClassMethods. It displays "hello world" in the console, important to remember that there is a console, and a chat in addition to in-game chat window(unless you removed it).